Multi-channel optical signals typically comprise a plurality of spectral channels, each having a distinct center wavelength and an associated bandwidth. The center wavelengths of adjacent channels are spaced at a predetermined wavelength or frequency interval, and the plurality of spectral channels may be wavelength division multiplexed to form a composite multi-channel signal of the optical network. Each spectral channel is capable of carrying separate and independent information. At various locations, or nodes, in the optical network, one or more spectral channels may be dropped from or added to the composite multi-channel optical signal, as by using, for example, a reconfigurable optical add-drop multiplexer (ROADM).
Reconfigurable optical add-drop architectures are disclosed in commonly assigned U.S. Pat. Nos. 6,549,699, 6,625,346, 6,661,948, 6,687,431, and 6,760,511, the disclosures of which are incorporated by reference herein.
An optical switching node may comprise one or more wavelength selective switches (WSS) configured as ADD and/or DROP modules. The referenced patents disclose wavelength selective switch apparatus and methods comprising an array of fiber coupled collimators that serve as input and output ports for optical signals, a wavelength-separator such as a diffraction grating, a beam-focuser, and an array of channel beam deflecting elements, one beam deflecting element for each spectral channel. In operation, a composite multi-wavelength optical signal (also referred to herein as a “multi-channel optical signal”) from an input port is supplied to the wavelength separator. The wavelength separator spatially separates or de-multiplexes the free-space multi-wavelength optical signal into an angular spectrum of constituent spectral channels, and the beam-focuser focuses the spectral channels onto corresponding ones of the channel beam deflecting element. By way of example, and not by way of limitation, the channel beam deflecting elements may be implemented in the form of micromirrors. The channel beam deflecting elements are positioned such that each channel beam deflecting element receives an assigned one of the separated spectral channel beams. The beam deflecting elements are individually controllable and continuously pivotal (or rotatable) so as to reflect the spectral channel beams into selected output ports. This enables each channel beam deflecting element to direct its corresponding spectral channel into any possible output port and thereby switch the spectral channel to any desired output port. Each output port may receive none, one, or more than one of the reflected and so directed spectral channels. Spectral channels may be selectively dropped from a multi-channel signal by switching the channels to different output ports, and new input channels may be selectively added or combined with the original channels to form different multi-wavelength composite signals.
It is also desirable, for a number of reasons, to be able to monitor and control the power in individual spectral channels of the multi-wavelength optical signal. This includes the ability to completely block the power contained in a particular spectral channel. One reason for controlling the power in a channel is to afford “hitless” switching to minimize undesired crosstalk during repositioning of a channel beam deflecting element to direct (“switch”) an input spectral channel beam to a desired output port. During repositioning, the channel beam deflecting element redirects the input spectral channel beam across, i.e., “hits”, intermediate ports, which couples unwanted light into the intermediate ports, and causes crosstalk. Thus, it is desirable either to completely block or to substantially attenuate the power in the beam during switching so that unwanted light coupling is avoided. Another use of monitoring and controlling the optical power of a channel is to afford attenuation of that channel to some predetermined level.
With the substantial growth of the demand for internet bandwidth, the internet traffic requirements have become quite unpredictable. In facing this challenge, the network has evolved to use ROADM (Reconfigurable Optical Add Drop Modules) at nodes in rings or mesh networks. These networks require dedicated wavelength selective switches (WSS). As shown in FIG. 1, the traffic from Point A to Point B can be routed dynamically. To enable routing flexibility, the system is likely to have many usable wavelengths or channels. When needed, a new channel is deployed in response to an increased bandwidth requirement of a particular node or is required from congestion/disruption of a part of the network. The evolution of WSS involves two basic architectures: Colored or Colorless. The former will switch a specific wavelength to its associated output fiber. The latter can switch a specific wavelength to any of the output fibers. The colored WSS typically uses AWG (Arrayed Waveguide Gratings) as the wavelength mux/demux element. The switching is performed between fibers or waveguides. The colored WSS is not flexible because fixed or specified wavelengths of the lasers are needed for the ADD module, even though tunable laser is widely available. The wavelength is fixed due to the physical association between the wavelength and the fiber. Once a fiber is connected to the laser, the wavelength is determined. Using the AWG based colored WSS the ROADM and thus the network is inflexible. It means the wavelength provision or routing is made when the lasers are installed, which is a manual operation.
The colorless WSS provides the freedom of choosing any wavelength dynamically, provided tunable lasers are connected to the ADD module. However, each tunable laser can only transmit data via one WDM channels. If more wavelengths are needed from a node, more tunable lasers will be needed to connect to the WSS. In this case, more WSS ports are needed. The more ports needed for the local add purpose, the fewer ports can be used between nodes. For this reason, it is desirable to have WSS with higher port counts.
However, there are many constraints to limit the number of ports in a WSS. The requirement hitless switching sets a topology challenge to the design of WSS. Currently, the port count of a free-space optical WSS is limited by the maximum angle that the micro-mirrors or light modulators can tilt. Micro-mirrors and light modulators are both considered a type beam deflecting element, (BDE). The allocation of angular range is determined by several factors. For instance, the packing density of the ports is limited by the number of resolvable spots. The latter is controlled by performance constraints on the port-to-port cross-talk, and hitless reconfiguration. Also, the waist of the optical beam at the BDE is usually smaller in one dimension to increase the passband width and this also reduces the number of resolvable spots in that dimension compared to the orthogonal direction. For those reasons, the WSS is typically designed with a 1×N configuration of ports at the input fiber collimator.
If the port count of WSS is greater than 25% of the number of wavelength channel, the network can be designed with good flexibility. If the number of port number of WSS is equal to the number of the wavelength channel, the network has a complete flexibility to drop all channels at one node and add all channels back. However, with the current state of art, the number of ports is typically about 10%. Therefore, it is desirable to increase the port count of WSS by 2 to 10 times.
It is within this context that embodiments of the present invention arise.